Agglomeration and charge loss sensor with seed structure for measuring particulate matter

ABSTRACT

A sensor includes an electrode and a seed structure. The electrode is configured to measure current due to movement of particulate matter relative to the electrode. The seed structure is deposited on the electrode. The seed structure includes a plurality of elongated members extending outward from the surface of the electrode. The elongated members are configured to promote charge transfer to particles and/or agglomerates of the particulate matter during operation of the sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/166,658 (Atty docket no. EMI-P030CIPDIV), which is a divisional ofU.S. application Ser. No. 13/853,890 (Atty docket no. EMI-P030CIP),which is a continuation-in-part of U.S. application Ser. No. 13/315,146(Atty docket no. EMI-P030), which claims the benefit of U.S. ProvisionalApplication No. 61/490,528, filed on May 26, 2011. Each of theseapplications is incorporated by reference herein in its entirety.

BACKGROUND

Internal combustion engines (e.g. diesel engines) typically generate anexhaust flow that contains varying amounts of particulate matter (PM).In general, particulate matter is small particles of solid matter (i.e.,soot) that are suspended in exhaust gasses. The amount and sizedistribution of particulate matter in the exhaust flow tends to varywith engine operating conditions, such as fuel injection timing,injection volume, injection pressure, or the engine speed to loadrelationship. Adjustment of these conditions may be useful in reducingparticulate matter emissions and average particle size in theparticulate matter from the engine. Reducing particulate matteremissions from internal combustion engines is environmentally favorable.In addition, particulate matter measurements for diesel exhaust isuseful for on-board (e.g., mounted on a vehicle) diagnostics of PMfilters and reduction of emissions through combustion control.

On-board (in situ) sensors for particulate matter typically fall intotwo categories:

1. Accumulative sensors

2. Real-time sensors

Accumulative sensors make use of the effect that exhaust particulates,especially soot particulates, tend to stick to surfaces exposed toexhaust gas. In an accumulative sensor, soot is allowed to accumulate onan exposed surface as a layer. A property of that soot layer ismeasured, either by vibrating the body (of which the surface is part) atits resonance frequency and then measuring the resonance frequencychange that results from the accumulation of soot, or by measuring theresistance of the accumulated soot or its capacitance. When a certainthreshold of the measured quantity is reached, the accumulation surfaceis heated to a high temperature to burn off the accumulated soot and theaccumulation process starts anew. The frequency of repetition of thisprocess is then used as a measure of the average particulate (or soot)content of the exhaust gas. Accumulative sensors are simple inconstruction, small enough to be installed like other exhaust sensorsand relatively inexpensive. However, they have, due to their operatingprinciple, a relatively slow response speed (not real-time) and sufferreliability issues when particles are accumulated that cannot be readilyburned off.

Real-time sensors have response speeds in the millisecond range andtypically make use of electrostatic effects on particulates, or useoptical effects.

These sensors fall into four categories:

a. Natural charge detectors

b. Ionizing induced charge sensors

c. Contact charge sensors

d. Optical sensors

Natural charge detectors try to detect the natural charge of particlesproduced during the production process. These sensors require verysensitive charge electronics and suffer from the fact that the naturalcharge of particles and/or their polarity can change on their waythrough the exhaust system.

Ionizing induced charge sensors create ions in the exhaust gas paththrough the sensor using a very high voltage on an electrode of a highsurface curvature, like a thin wire or needle tip. Voltages aretypically in the 2-15 kV range. This voltage causes a corona dischargein the particle carrying gas. Soot particles flowing through the coronadischarge field acquire an electric charge. These charged particles arethen collected by a collection electrode and the charge transfer rate ofthe collection electrode is measured. The collection electrode and thenecessity to prevent charged gas ions to also transfer a charge to thecollection electrode requires a fairly large and complicated apparatusthat cannot be easily reduced to the size of a typical exhaust sensor.

Contact charge sensors typically use much lower voltages than ionizinginduced charge sensors. In contact charge sensors soot particles comingin contact with a high voltage electrode acquire a surface charge thatis determined by the surface charge density of the high voltageelectrode. Typical voltages for the high voltage electrode are in the500V to 3 kV range. These charged particles then deposit their acquiredcharge to grounded parts of the exhaust system or to a secondarydetection electrode. The charge loss from the high voltage electrode istypically proportional to the particle concentration in the exhaust gasand is measured. However, because the resulting current (charge transferper second) is very small, it is very difficult to isolate the highvoltage electrode sufficiently to prevent current leakage. Any currentthat flows through an imperfect isolation to ground also creates acharge loss on the high voltage electrode and therefore causes a falsesensor signal. As a way around that problem, a second collectionelectrode, essentially at ground voltage level, is placed in closeproximity to the high voltage electrode and the charge accumulation onthat electrode is measured. However, this necessitates an additionalelectrical connection that has to be well insulated from the highvoltage supply to the electrode. For the typical temperaturesencountered in the exhaust system of internal combustion engines it isvery hard to find insulating materials that can withstand thosetemperatures and still maintain the high electrical insulatingproperties required.

The current detected by contact charge sensors is proportional to theparticle content of the exhaust gas, but also proportional to the areaof the electrodes. For the areas possible for a typically sized exhaustsensor the currents are in the low picoAmpere range for the typicallyencountered soot levels, which are very difficult to detect in theelectrically noisy environment where internal combustion enginesoperate. Furthermore, the low currents require the use of electrometergrade amplifiers to detect and amplify the sensor signal. Due tolimitations of current semiconductor technology, these amplifierstypically can maintain their specifications only over a narrowtemperature range, which is much smaller than the typical temperaturerange required for vehicular applications (typically −40 to +125 degreesCelsius).

Optical sensors consist of a light source and a light detector. Theymeasure either the opacity of the gas stream containing particles ormeasure light that is scattered by particles in the light path of thelight source.

Common to all described real-time sensor methods is that sootaccumulation on the sensor parts has a detrimental effect on the sensorperformance and it is attempted to be remedied by various methods,depending on the embodiment. Either by diluting the exhaust gas withclean air, flowing compressed filtered air periodically past theelectrodes (or optical parts in case of optical sensors) to blow offaccumulated particles, or by heating the electrodes (or lenses in caseof optical sensors) to a temperature where accumulated soot particlesburn off, but not high enough to burn contacting soot particlesimmediately.

Further common to the described real-time sensors is that they havereaction times in the low millisecond range and can therefore forexample detect changes in the soot concentration of the exhaust gas on acylinder by cylinder basis for internal combustion engines.

Because of the above described limitations of the state-of-the-artparticulate sensors for in-situ applications there is a need for asensor that overcomes some of the limitations.

The sensor described in this invention combines certain aspects of theaccumulative and the contact charge sensors in such a way as to increasethe measured current by several orders of magnitude compared to acontact charge sensor, and by requiring soot accumulation on theelectrodes to operate.

This allows this sensor to be scaled to the size of a typical exhaustsensor and makes it possible to use common insulators for theelectrodes. In addition the sensor described in this invention does notrequire any special remedies to prevent soot accumulation on the sensingelectrode. Compared to other real-time sensors, the sensor described inthis invention has a slower response time, typically slower than 100milliseconds, but faster than 5 seconds, which is sufficient for mostapplications.

SUMMARY

Embodiments of an apparatus are described herein. In one embodiment, theapparatus includes a sensor to measure particulate matter in an exhaustgas. An embodiment of the sensor includes an electrode and a seedstructure. The electrode is configured to measure current due tomovement of particulate matter relative to the electrode. The seedstructure is deposited on the electrode during manufacturing orotherwise prior to use of the electrode. The seed structure includes aplurality of elongated members extending outward from the surface of theelectrode. The elongated members are configured to promote chargetransfer to the particulate matter during operation of the sensor. Thecharge transfer may be to particles and/or agglomerates of theparticulate matter. In one embodiment, the seed structure includesdendric rhenium disposed on the surface of the electrode. In anotherembodiment, the seed structure includes nano-wires disposed on a surfaceof the electrode. Other embodiments may include other types of elongatedmembers with similar size, geometry, chemical properties, and/or otherfeatures similar to dendric rhenium or nano-wires. Other embodiments ofthe sensor are also described.

Embodiments of methods are also described herein. In one embodiment, amethod for making a sensor includes providing an electrode for measuringcurrent due to movement of particulate matter relative to the electrode.The method also includes depositing a seed structure on the electrode.The seed structure includes a plurality of elongated members extendingoutward from the surface of the electrode. The elongated members areconfigured to promote charge transfer to the particulate matterparticles and/or agglomerates during operation of the sensor.

In another embodiment, a method for using a sensor includes passingexhaust by an electrode having a seed structure deposited thereon. Theseed structure includes a plurality of elongated members extendingoutward from the surface of the electrode. The elongated members areconfigured to promote charge transfer to the particulate matter duringoperation of the sensor. The method also includes measuring current dueto movement of particulate matter relative to the electrode. The methodalso includes correlating the measured current to a quantity of theparticulate matter in the exhaust. Other embodiments of methods are alsodescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic block diagram of one embodiment of an exhaustsensor system.

FIG. 2A depicts a schematic diagram of an exterior of one embodiment ofthe sensor assembly of FIG. 1.

FIG. 2B depicts a schematic diagram of an interior of one embodiment ofthe sensor assembly of FIG. 1.

FIG. 2C depicts a schematic diagram of a flow pattern through theinterior of the sensor assembly of FIG. 2B.

FIG. 2D depicts a schematic diagram of another embodiment of the sensorassembly of FIG. 1.

FIG. 3 depicts a schematic circuit diagram of one embodiment of thecontrol circuit of FIG. 1.

FIG. 4 depicts a schematic circuit diagram of another embodiment of thecontrol circuit of FIG. 1.

FIG. 5 depicts a schematic circuit diagram of the sensor assembly ofFIG. 1.

FIG. 6A depicts a schematic circuit diagram of one embodiment of asensor assembly circuit.

FIG. 6B depicts a schematic circuit diagram of another embodiment of asensor assembly with a guard trace.

FIG. 7 depicts an exploded view of one embodiment of a heater assemblyfor use in a sensor assembly such as the sensor assembly of FIG. 2D.

FIG. 8A depicts a perspective side view of an assembled embodiment ofthe heater insulator assembly of FIG. 7.

FIG. 8B depicts another perspective side view of an assembled embodimentof the heater insulator assembly of FIG. 7.

FIG. 9 depicts results from modeling of one embodiment of a sensorassembly circuit which includes a guard trace.

FIG. 10 depicts one embodiment of a sensor electrode with a depositedseed structure of, for example, dendritic rhenium.

FIG. 11A depicts an enlarged view of one embodiment of the seedstructure of FIG. 10.

FIG. 11B depicts an enlarged view of another embodiment of the seedstructure of FIG. 10 having additional particle depositions connected tothe seed structure.

FIG. 11C depicts a further enlarged view of the seed structure of FIG.11B.

FIG. 12 depicts one embodiment of a sensor electrode with a coupledstructure of nano-wires.

FIG. 13 depicts an enlarged view of another structure of nano-wires.

Throughout the description, similar reference numbers may be used toidentify similar elements.

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments asgenerally described herein and illustrated in the appended figures couldbe arranged and designed in a wide variety of different configurations.Thus, the following more detailed description of various embodiments, asrepresented in the figures, is not intended to limit the scope of thepresent disclosure, but is merely representative of various embodiments.While the various aspects of the embodiments are presented in drawings,the drawings are not necessarily drawn to scale unless specificallyindicated.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by this detailed description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the present invention should be or are in anysingle embodiment of the invention. Rather, language referring to thefeatures and advantages is understood to mean that a specific feature,advantage, or characteristic described in connection with an embodimentis included in at least one embodiment of the present invention. Thus,discussions of the features and advantages, and similar language,throughout this specification may, but do not necessarily, refer to thesame embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize, in light ofthe description herein, that the invention can be practiced without oneor more of the specific features or advantages of a particularembodiment. In other instances, additional features and advantages maybe recognized in certain embodiments that may not be present in allembodiments of the invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the indicatedembodiment is included in at least one embodiment of the presentinvention. Thus, the phrases “in one embodiment,” “in an embodiment,”and similar language throughout this specification may, but do notnecessarily, all refer to the same embodiment.

It will be readily understood that the components of the embodiments asgenerally described herein and illustrated in the appended figures couldbe arranged and designed in a wide variety of different configurations.Thus, the following more detailed description of various embodiments, asrepresented in the figures, is not intended to limit the scope of thepresent disclosure, but is merely representative of various embodiments.While the various aspects of the embodiments are presented in drawings,the drawings are not necessarily drawn to scale unless specificallyindicated.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by this detailed description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope. Reference throughout thisspecification to features, advantages, or similar language does notimply that all of the features and advantages that may be realized withthe present invention should be or are in any single embodiment of theinvention. Rather, language referring to the features and advantages isunderstood to mean that a specific feature, advantage, or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the present invention. Thus, discussions of the featuresand advantages, and similar language, throughout this specification may,but do not necessarily, refer to the same embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize, in light ofthe description herein, that the invention can be practiced without oneor more of the specific features or advantages of a particularembodiment. In other instances, additional features and advantages maybe recognized in certain embodiments that may not be present in allembodiments of the invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the indicatedembodiment is included in at least one embodiment of the presentinvention. Thus, the phrases “in one embodiment,” “in an embodiment,”and similar language throughout this specification may, but do notnecessarily, all refer to the same embodiment.

The described embodiments include a particulate matter sensor thatoperates based on the agglomerated charge loss principle.

An exhaust stream containing small particles is directed toward anelectrode connected to the positive connection of an insulated highvoltage power supply. These particles will naturally attach themselvesto the high voltage electrode and form a layer on that electrode. When,after a few minutes of operation, enough particles are attached, surfaceirregularities on the electrode and on the accumulated particle layeract as nucleation points where particles preferentially attach.

The time delay until which enough of a layer is formed depends on theparticle concentration in the exhaust. Typically it is between 5 and 10minutes for typical soot concentrations in the exhaust of a dieselengine. However, this time delay is relevant only for a new electrodethat has never been exposed to exhaust gas. The soot layer initiallyformed will stay permanently on the electrode and can also beartificially created during the production process of the sensor toprevent this initial delay.

Because of the high electric field, and because the field strength, dueto the physical geometry of the sensor, these particles attachpreferentially to nucleation sites where the field strength isstrongest, typically a surface imperfection or roughness element. Sootparticles then continue to deposit in the presence of the electric fieldand form diffusion-limited aggregates (fractal or dendrite structures).Once the soot aggregates reach a large size (microns), dielectrophoresismay play a role whereby particles suspended in a fluid or gas will movetoward the stronger field area in an inhomogeneous electric field,provided their relative permittivity is higher than that of thesurrounding medium. The relative permittivity of soot or carbon istypically 5-10, while the relative permittivity of exhaust gas is closeto 1. Soot or carbon particles comprise the majority of particles in theexhaust of an internal combustion engine at operating temperature.

When these soot particles attach to the nucleation points, they attachand are held there by intermolecular forces, like Van der Waals forces.Once they contact, and if they are at least partially conductive, theyacquire the same surface charge as the electrode surface.

Because equal polarity charges repel, they are at the same time repelledby the electrode. However, for individual particles the electric fieldstrength used in this sensor (400-1500 kV/m) is not strong enough toovercome the attachment forces.

However, because of the electric field, soot particles will agglomerateinto branched filamentous 3-dimensional structures extending away fromthe electrode. Under the influence of the field these structures tend toself-organize such that they maximize their surface area as eachindividual branch of the structure acts as new nucleation and attachmentpoint for new particles, but at the same time each branch is repelled bythe other branches as they have equal charge and polarity. After sometime such a structure acquires enough surface charge such that theresulting repelling electrostatic force between the structure and theelectrode soot layer is enough to overcome the attachment force at thepoint where the structure is attached to the layer on the electrode, orin the structure where its attachment forces are weakest.

When these structures break off from the electrode, they carry part ofthe charge of the electrode with them and deposit their charge when theycontact a grounded part of the sensor or the exhaust system. This chargedeposition on the grounded part of the sensor can be detected as acurrent pulse flowing between electrode and ground, and is in thisembodiment measured by a current meter between ground and the negativeconnection of the high voltage power supply.

Because of the self-organization of the branched structures in theelectric field, these structures can carry a far higher charge per massunit than individual particles can, given the voltages used. In theembodiment described here, the ratio of charge per mass unit is 100-1000times higher than what would be expected from a charge transfer byindividual particles as used in a contact charge sensor. Otherembodiments may have different charge per mass ratios relative to thecharge transfer by individual particles.

The surface to mass ratio is implied from the measurements of current,particle mass flow, and particle size and is a virtual expression forcharge per mass unit acquired. The argument for high surface to massratio, or high surface charge density to mass ratio, is based on thefact that soot particles are conductive, therefore they can carry chargeonly on their surface.

During one example measurement, the mass concentration of particles was6 mg/m³ (particle mass per exhaust gas volume).

The measured particle count was about 6*10⁷ particles/cc and the averagediameter of the particles was 50 nm. The measured current was about 41nA, and the gas flow rate through the sensor was about 47 cc/second.This means about 2.82*10⁹ particles are flowing through the sensor witha total particle mass flow rate of about 282 ng/second.

The total surface area of the particles flowing through the sensor,assuming roughly spherical particles is therefore about 2.21*10⁻⁵m²/second. However, when they touch the electrode, what counts is notthe total surface area of the particles, but roughly their crosssectional area. Therefore, the approximate effective area is only5.5*10⁻⁶ m²/second for spherical particles.

The effective surface to mass ratio of those particles is thereforeabout 19.5 m²/g.

The surface charge density of the electrode with the described sensor at1000V is about 6.9*10⁻⁵ Coulomb/m².

If each particle acquires from the electrode the same surface chargedensity as the electrode itself has, and transfers that charge, therewould be a current of about 3.8*10⁻¹⁰ Coulomb/second or 0.38 nA. Thiscan be expressed also as charge transferred per mass unit which would bein this case about 1.35*10⁻³ Coulomb/g.

In one embodiment, the current actually measured with the sensor isabout 41 nA.

For a current of about 41 nA and assuming the agglomerates are acquiringthe same surface charge density as the electrode, the total surface areaof agglomerates transferring charge away from the electrode musttherefore be about 5.9*10⁻⁴ m²/second. Because the total particle massflowing through the sensor is not changing, therefore the averageeffective surface to mass ratio for the agglomerates is about 2092 m²/g.

Expressed as charge transferred per mass unit in this case comes toabout 0.15 Coulomb/g. However it's expressed, there is typically morethan a two orders of magnitude difference. Other embodiments may exhibita difference between about 1.5-5 order of magnitude, or greater.

In operation, for a given particle concentration, an equilibrium isreached where the rate of break-off of agglomerated structures and theirbuildup balance. Higher soot concentrations will build structuresfaster, therefore creating more and larger current pulses.

A larger surface area of the electrode also creates more opportunity tocreate more structures and therefore creates more current pulses.

However, the surface area of the electrode has preferentially a positivecurvature to create an inhomogeneous field. This physically limits thesize of the electrode. Some embodiments may use multiple smaller curvedelectrodes to create a larger surface area.

The current pulses are integrated in the electronics and yield anaverage overall current that is proportional to the particleconcentration of the flowing exhaust gas.

Increasing the voltage will create a higher surface charge density onthe electrode and the agglomerating structures, yielding current pulseswith higher average amplitude, but the limit of the voltage is the onsetof ionizing discharge from the tips of the agglomerated structures,which would create a current from the electrode to ground that is notproportional to the soot concentration.

The magnitude of this voltage has to be experimentally determined,depending on the specifics of the embodiment.

Because of the far higher current, this measurement method has far lowerdemands on the electrical insulation properties of the electrodemounting than a contact charge sensor would need. In addition thesensitivity of the signal amplification circuit can be reduced such thatit is possible, without expensive shielding, to deploy it in theelectrically noisy environment of a vehicle.

FIG. 1 depicts a schematic block diagram of one embodiment of an exhaustsensor system 10. The illustrated exhaust sensor system 10 includes asensor assembly 12, an engine 14, and an exhaust system 16. The engine14 produces exhaust which moves through the exhaust system 16. Theexhaust system 16 facilitates flow of the exhaust gases to a gas outlet18, typically for emission into the atmosphere. The sensor assembly 12is at least partially inserted into the exhaust system 16 to detect aparameter within the exhaust stream. As the gas in the exhaust system 16passes over and/or through the sensor assembly 12, the sensor assembly12 detects a condition within the exhaust by measuring chemicals ortemperature or other parameters at the sensor assembly 12, as describedherein. In a specific embodiment, the sensor assembly 12 includes aparticulate matter sensor to detect conditions indicative of thepresence of particulate matter within the exhaust stream.

The exhaust sensor system 10 also includes an electronic control module20. The electronic control module 20 includes a processor 22, and anelectronic memory device 24. The electronic control module 20 also mayinclude a control circuit 26 to control some or all of the operations ofthe sensor assembly 12. Alternatively, some or all of the controlcircuit 26 functionality may be implemented at the sensor assembly 12 orat another location that is not necessarily proximate the electroniccontrol module 20. Additionally, in some embodiments, the controlcircuit 26 may control a peripheral system (not shown). Some examples ofperipheral systems that may be implemented at the sensor assembly 12include, but are not limited to, a heater (not shown) or a chemicalneutralizer system (not shown). Instead of or in addition to thechemical neutralizer system, some embodiments may include an emissioncontrol element (not shown) to neutralize other aspects of the chemicalsand/or substances within the exhaust system 106, either upstream ordownstream from the sensor assembly 10. In other embodiments, thecontrol circuit 26 may control peripheral systems at other locationswithin the exhaust sensor system 10.

In one embodiment, the sensor assembly 12 relays a sensor signal to theprocessor 22 of the electronic control module 20. The processor 22analyzes the sensor signal from the sensor assembly 12. If the sensorsignal is corrupted, the processor 22 may send a control signal to thecontrol circuit 26, for example, to shut down the sensor assembly 12. Inthis situation, or in other situations, the control circuit 26 mayactivate one or more heaters inside of or within proximity to the sensorassembly 12 to burn off particulate matter deposits that might corruptthe sensor signal from the sensor assembly 12. In some embodiments, theprocessor 22 sends the control signal to the control circuit 26 toactivate a chemical injection system to introduce a chemical agent intothe exhaust system 16 to remove particulate matter deposits built up onthe sensor assembly 12. Further functionality of embodiments of thecontrol circuit 26 is described below.

If the sensor signal from the sensor assembly 12 is not corrupt, theprocessor 22 may compare the sensor signal with data stored in a lookuptable 28 on the electronic memory device 24 to determine one or morequalities of the exhaust in the exhaust system 16. For example, theprocessor 22 may determine an amount of particulate matter in theexhaust stream. The processor 22 also may compare the sensor signal fromthe sensor assembly 12 with data from the lookup table 28 to estimate,for example, a mass concentration of particulate matter in the exhauststream. In other embodiments, the electronic control module 20facilitates detection of one or more other qualities of the gas in theexhaust system 16. For example, types of sensors that may detectqualities of the exhaust stream may include but are not limited to, aparticulate matter sensor, an oxygen sensor, a thermal sensor, anammonia sensor, a flow rate sensor, and an air-fuel ratio sensor.

It should also be noted that embodiments of the sensor assembly 10 maybe tolerant of fluctuations of certain gaseous constituents in a gasenvironment. In this way, the sensor assembly 10 may be calibrated tomeasure particular chemicals, materials, or other conditions within anexhaust stream, with relatively little or no disruption from one or moreother chemical substances and/or operating conditions.

It should also be noted that the sensor assembly 12 may be used, in someembodiments, to determine a failure in another component of the exhaustsensor system 10. For example, the sensor assembly 12 may be used todetermine a failure of a particulate matter filter (not shown) withinthe exhaust system 16. In one embodiment, a failure within the exhaustsensor system 10 may be detected by an elevated signal generated by thesensor assembly 12. In some embodiments, the exhaust sensor system 10includes an alarm to indicate a detected failure of the sensor assembly12 or other component of the exhaust sensor system 10. In someembodiments, the sensor assembly 12 also could be coupled to anothersensor or detector such as a mass flow meter.

FIG. 2A depicts a schematic diagram of an exterior of one embodiment ofthe sensor assembly 12 of FIG. 1. FIG. 2B depicts a schematic diagram ofan interior of one embodiment of the sensor assembly 12 of FIG. 2A. Theillustrated external components of the sensor assembly 12 include asensor housing 120, an intermediate bolt portion 122, and a threadedportion 124. The depicted external components also include an outerhousing 126 with a cap 128. The outer housing 126 has one or more holes130 which provide airflow within the outer housing 126. The cap 128includes a separate opening 132 to facilitate the airflow within theouter housing 126. One example of an airflow pattern through the outerhousing 126 of the sensor assembly 12 is shown in FIG. 2C and describedin more detail below.

For reference, the sensor assembly 12 is installed within the exhaustsensor system 100 so that the sensor housing 120 and the bolt portion122 of the sensor assembly 12 are typically outside of the exhauststream through the exhaust system 106. The outer housing 126 and the cap128 of the sensor assembly 12 are installed within the exhaust streamthat passes through the exhaust system 106. The dashed line 134distinguishes, generally, between the parts of the sensor assembly 12that are outside (left of the dashed line 134) of the exhaust system 106and the parts of the sensor assembly 12 that are inside (right of thedashed line 134) of the exhaust system 106. In one embodiment, thethreaded portion 124 allows the sensor assembly 12 to be threaded, orscrewed, into a corresponding hole within the exhaust system 106. Someof the threaded portion 124 may remain outside of the exhaust system 106or, alternatively, may enter into the exhaust system 106. Many of theexternal components of the sensor assembly 12 may be made from metals,such as stainless steel, that are substantially insensitive to typicalmechanical and/or chemical conditions within the exhaust sensor system100.

In one embodiment, as exhaust gasses flow by the cap 128, the associatedVenturi effect creates a low pressure which draws exhaust gas out of theinterior of the outer housing 126. A corresponding amount of exhaust gasis drawn from the ambient exhaust stream through the holes 130 into theouter housing 126. Drawing a portion of the ambient exhaust stream intothe outer housing 126 allows a sensor electrode 136 to measure theamount of particulate matter (PM) within the exhaust stream.

In one embodiment, the large surface area of the electrode 136 iscylindrically shaped. The cylinder can be hollow for weight reduction.The axis of the cylinder may be coincident with the axis of theelectrode mounting stem 138.

FIG. 2B shows the inner construction of one embodiment of the sensorassembly 12. The inner construction of the sensor assembly 12 includesthe sensor electrode 136 and the electrode mounting stem 138, as well asan inner baffle 140 and one or more insulators 142 and 144. In thedepicted embodiment, the electric field extends radically between thesurface of the electrode 136 and inner baffle 140. Because the electrode136 has a smaller outside diameter than the inside diameter of innerbaffle 140, the electric field is stronger at the surface of theelectrode 136. In general, exhaust gas passing by the sensor assembly 12enters into the outer housing 126 through the exhaust inlet holes 130.The exhaust gas flows through the space between inner baffle 140 and theouter housing 126 to one or more baffle holes 146 at the base of theinner baffle 140. The gas then flows in the space between cylindricalsensor electrode 136 and the cylindrical inner baffle 140 to the exhaustoutlet 132 in the Venturi cap 128.

Because the flow of exhaust gas has to make a 90 degree turn whenflowing from the holes 146 in the inner baffle into the space betweenthe inner baffle 140 and the electrode 136, particles suspended in thegas stream are accelerated towards the electrode 136 because of theirhigher inertia compared to gas molecules. This flow pattern furthersupports the formation of particle agglomerates on the electrode 136.

In one embodiment, the inner baffle 140 is metallic and connected to theground. The Venturi cap 128 is also connected to the inner baffle 140and grounded.

Agglomerates that detach from the electrode 136 are repelled by theelectrode 136 due to having equal charge and are attracted by thesurface of the inner baffle 140, which has an opposite surface charge tothe electrode 136 or agglomerates. Because detached particleagglomerates flow with the gas stream between inner baffle 140 andelectrode 136, and because the gas stream has to execute two more turnsto exit through the Venturi hole 132, these turns in the flow furtherinsure that even agglomerates that have not deposited their charge inthe inner baffle 140 are accelerated by their inertia towards a groundedsurface part of the Venturi cap 128.

The grounded filter baffle 144 and the high voltage filter baffles 148,which are disk shaped, provide a tortuous path for particles that couldmigrate to the electrode insulator 142. The electric field between thesefilter baffles is highly inhomogeneous at the inner edge of filterbaffle 144 and at the outer edges of filter baffles 148. Therefore, anyparticulates that migrate along the path between those filter bafflesare attracted to these filter baffles themselves and will attach there.The normal gas flow path through the sensor insures that there will notbe enough soot particles migrating to create large structures. Thefilter baffles therefore slow down particle accumulation on theinsulator 142. Particle accumulations on the insulator 142 could form anelectrically conductive path that will create a charge loss fromelectrode 136 to the sensor housing 120, parallel and in addition to thecharge loss by particle agglomerates, which could detrimentally affectsensor performance. In addition, in some embodiments, the insulator 142could be periodically heated by an embedded heater (not shown) such thatany accumulated soot particles on the surface of the insulator 142 areburned off.

In some embodiments, this burn-off of soot particles may be implementedinfrequently. Even in exhaust gas with particle concentrations far abovethe legal limit for emissions, the burn-off may be required only everyfew hours of operation.

FIG. 2D depicts a schematic diagram of another embodiment of the sensorassembly 12 of FIG. 1. In contrast to the sensor assembly of FIG. 2B,the sensor assembly of FIG. 2D includes a heater assembly 162 (alsoreferred to as a heater disk 162). The heater assembly 162 may performsome or all of the insulating functions of the insulator 144 of FIG. 2B.Additionally, the heater assembly 162 may perform one or more of theheater functions described herein. A more detailed depiction of theheater insulator assembly 162 is shown in FIGS. 7, 8A, and 8B anddescribed in more detail below. The illustrated sensor assembly of FIG.2D also includes a heater bushing 164, and separate electrical leads 166and 168 for the heater and ground traces (see FIGS. 7, 8A, 8B). Ingeneral, the heater may be controlled to provide heat to the adjacentelectrically insulating surfaces so as to burn off deposited particlesthat may have settled on or otherwise coupled to the insulatingsurfaces. Additionally, the heater supply heat to the sensor electrode136, or a portion thereof, to burn off some or all of the soot ordeposited particles at the heated location(s). The heater bushing 164provides an insulated pathway for the corresponding portion of thesensor electrode 136 to pass through the heater insulator assembly 162.

In operation, exhaust gas flowing past the Venturi tip 128 of the sensorassembly 2 creates a low pressure area at the tip. Flow through thesensor assembly 12 is also promoted by a positive pressure that iscreated at the “inlet holes” 130 because of the velocity head (velocityis reduced to zero at the inlet holes and velocity is converted topressure at the inlet holes by Bernoulli's principle). The low pressureat the tip 128 combined with the positive pressure at the inlet holes130 draw exhaust gas through the sensor in a reproducible manner. Theexhaust gas flows up through the space between exhaust housing and innerbaffle and through holes 146 in the inner baffle 140 into the spacebetween inner baffle 140 and electrode 136 and back down towards theVenturi hole 132. Although different operating conditions may beachieved, on example of a model of the illustrated embodiment exhibits afree stream exhaust velocity of 14 m/s and an exhaust gas temperature(EGT) of 200 C. At these conditions, the velocity in the sensor is 0.34m/s (Re=25), and the volumetric flow rate is 0.82 liters/minute.

In one embodiment, the sensor body, including the inner baffle 140, isgrounded through the vehicle exhaust system to the vehicle ground (notshown). The cylindrical electrode 136 is held at a potential ofapproximately 1,000 V relative to ground. In the depicted embodiment,the electrode 136 and its connections are insulated with two aluminadiscs: the back insulator disk 142, which also acts as holder for theheater and guard connections, and the heater disk 162. The heater disk162 also mechanically fixes the electrode 140 assembly in the sensor 12.In one embodiment, one or both of the insulator disks 142 and 162 areco-fired. In some embodiments, the heater disk 162 contains a platinumthick-film heating element (see FIGS. 7, 8A, 8B) that serves also asresistance temperature detector (RTD) for temperature measurements. Theheater element can be used to evaporate moisture that condenses on theheater disk 162 during condensing conditions, and as mentioned above canalso be used to burn off soot that accumulates on the exhaust side faceof the heater disk 162 over time. Under some operating conditions thefunction of burning off soot may be used infrequently, as needed.

In one embodiment, the electrode 140 assembly is brazed to the heaterbushing 164, which acts as the main insulator between the electrode 136and heater disk 162. The hole in the heater disk 162 in which the heaterbushing 164 is brazed is surrounded on both sides of the heater disk bymetalized guard rings (see FIGS. 7, 8A, 8B). These guard rings areelectrically connected to the brazing metal in the heater disk 162(e.g., to a heater bushing braze joint) and connected to the “guard”connection 166. Similar guard rings may be implemented on one or both ofthe outermost surfaces of the heater disk 162 (See FIGS. 8A and 8B).These guard traces are further connected to the inner shield of atriaxial sensor supply cable (guard), while the inner conductor of thetriaxial cable supplies the high potential for the electrode 136. Theouter conductor of the triaxial cable is connected on the electronicside to ground to provide RF and capacitive noise shielding.

The electronics for the sensor 12 hold the guard connection at groundpotential while the guard itself is connected to the negative side ofthe 1,000 V supply. Therefore, any current leakage through either thebulk resistivity of the heater bushing 164, or due to conducting surfacecontaminants on heater disk 162 or back insulator between the electrode136 and the guard trace creates a current load on the HV power supply,but does not contribute to a current measured between the negative ofthe 1,000 V supply and ground.

FIG. 3 depicts an electrical schematic block diagram of one embodimentof the control circuit of FIG. 1.

The positive connection of the high voltage source 104 is connected tothe electrode 108 of sensor assembly 12, while the inner baffle 140 andthe rest of the sensor assembly 12 are connected to ground. This rest ofthe sensor assembly 12 is depicted as part 110 in the electricalschematic block diagram. The negative connection of the high voltagesource 104 is connected to one side of the filter resistor 114 and toone side of the filter capacitor 112. The other side of filter capacitor112 is grounded. The other side of the resistor 114 is connected to oneside the current meter 106, and the other side of the current meter 106is grounded.

The resistor 114 and filter capacitor 112 form a low pass filter with abandwidth of, for example, 5-10 Hz. The current pulses created by theoperation of sensor assembly 12 are integrated in this low pass filter.

FIG. 4 depicts a more detailed electrical schematic of the schematicblock diagram depicted in FIG. 3. Generally, the illustrated controlcircuit 26 includes a generator block 150, a protection block 152, and adetection and filter block 154. Other embodiments may include fewer ormore blocks and/or fewer or more components within one or more of theillustrated blocks.

In the depicted embodiment, the generator block 150 includes an impulsegenerator (IG), a bi-directional Zener diode (Z1), a transformer (Tr), ahigh voltage diode (D1), and a high voltage capacitor (C1). Theillustrated transformer includes primary and secondary windings with a1:10 winding ratio, although other embodiments may have differentwinding ratios. The protection block 152 includes the clamping diode D2.The detection block 154 includes an initial filter resistor (RF), aninitial filter capacitor (CF), an operational amplifier (OA), a gainresistor (RG), a PMOS transistor (Q), and a current to voltageconversion resistor (RS).

In general, the control circuit 26 generates a relatively high voltageto apply to the sensor electrode 108 and, in turn, generates an outputsignal that can be correlated with the particulate matter level of theexhaust stream to which the sensor electrode 108 is exposed.Specifically the secondary side of the transformer TR, diode D1, andcapacitor C1 form an embodiment of the floating voltage source 108depicted in FIG. 3. While specific circuit components are shown in aparticular arrangement, other embodiments may use similar or differentcircuit components to achieve the same or similar results. Additionally,while the illustrated circuit is implemented substantially in hardware,it may be possible to implement some portions of the control circuit 26using software instructions that are executed by a central processor orother digital signal processing device.

In one embodiment, the impulse generator 156 generates periodic, shortpulses duration (e.g., about 1-2 μsec) with a particular repeat rate(e.g., about 1400 impulses per second) and a maximum amplitude (e.g.,about 1000V). These impulses charge the capacitor C1 to the sameamplitude (e.g., about 1000V). The positively charged side of capacitorC1 is connected to the sensor electrode 108 of the sensor assembly 100.Agglomerates that get positively charged at the sensor electrode 108carry away some of the capacitor charge periodically and, thus,discharge the capacitor C1 relative to the ground reference (e.g.,vehicle ground). This discharge current flows through the initial filterresistor RF and are integrated at the initial filter capacitor CF. Theoperation amplifier OA together with the gain resistor RG and PMOStransistor Q form an inverting current amplifier with a gain of—RF/RG.This amplified current Tout flows out of the drain of PMOS transistor Qinto the voltage conversion resistor RS. The voltage at resistor RS istherefore proportional to RS*Iout, according to Ohms law. Other methodsof detecting the discharge current can be employed as well.

In one embodiment, the impulse generator 156 receives vehicle batteryvoltage (about 12V) and switches the vehicle battery voltage on theprimary winding of the transformer until the current in this windingreaches a primary winding threshold value. In one embodiment, theprimary winding threshold value is approximately 3 Amperes, althoughother embodiments may use a different primary winding threshold value.When the specified current limit is reached, the current is rapidlyswitched off. In one embodiment, the inductive flyback pulse of theprimary winding is limited by the bidirectional Zener diode Z1 to avalue of 100V. Because the transformer has a winding ratio of 10:1between its secondary and primary windings, the flyback pulse limited to100V on the primary side translates into a 1000V pulse on the secondaryside. Although some embodiments may use 100V on the primary side and1000V on the secondary side, other embodiments may use differentvoltages and/or different winding ratios.

The impulse generator 156 periodically switches the battery on to theprimary winding to generate corresponding pulses on the secondary sideon a regular basis. In one embodiment, pulses are generated once every0.7 msec (1.4 kHz), although other embodiments may use a different pulsegeneration frequency.

The 1000V pulse on the secondary side of the transformer charges thehigh voltage capacitor C1 via the diode D1 to 1000V. This circuit is aflyback converter with primary voltage limiting.

The 1000V charge of the high voltage capacitor C1 is connected to thesensor electrode 108.

If soot agglomerates extend all the way from the sensor electrode 108 tothe grounded part 110 of the sensor assembly, a short circuit would formthat shorts the high voltage source to ground. This creates a highcurrent that would discharge C1 rapidly while CF would be chargedrapidly to a high negative voltage. The protection diode D2 prevents CFto be charged to a more negative voltage than about −0.7V. The highdischarge current of C1 will heat up the particles of which the shortingsoot agglomerate is formed to a high enough temperature to burn them.This way these types of short circuits are self extinguishing.

The negative side of high voltage capacitor C1 is connected to theinitial filter resistor RF and initial filter capacitor CF. The filtercapacitor CF integrates the current pulses caused by dislodged sootagglomerates as described before. Therefore the average current throughRF is proportional to the charge loss rate from the electrode caused bydislodging soot agglomerates depositing their charge to a grounded partof the sensor or exhaust system. This integrated current is correlateswith the soot concentration of the exhaust gas flowing through thesensor. Because of the need for integration of the current pulses, thissensors response time is determined by the low pass filter bandwidth ofthe low pass filter formed by RF and CF. For typical implementations,this bandwidth is less than 10 Hz.

The exhaust sensor system 10 also includes an electronic control module20. The electronic control module 20 includes a processor 22, and anelectronic memory device 24. The electronic control module 20 also mayinclude a control circuit 26 control some or all of the operations ofthe sensor assembly 12. Alternatively, some or all of the controlcircuit 116 functionality may be implemented at the sensor assembly 12or at another location that is not necessarily proximate the electroniccontrol module 20. Additionally, in some embodiments, the controlcircuit 26 may control a peripheral system (not shown). Some examples ofperipheral systems that may be implemented at the sensor assembly 12include, but are not limited to, a heater (not shown) or a chemicalneutralizer system (not shown). Instead of or in addition to thechemical neutralizer system, some embodiments may include an emissioncontrol element (not shown) to neutralize other aspects of the chemicalsand/or substances within the exhaust system 106, either upstream ordownstream from the sensor

FIG. 5 depicts a schematic circuit diagram of the sensor assembly ofFIG. 1. The illustrated circuit diagram includes a processor 172, a highvoltage generator 174, a sensitive current meter 176, a current meter178, and a heater drive 180 to control the heater. The illustratedcircuit diagram also includes several control and/or communicationlines. In one embodiment, the electrode 136 is connected to the positiveside of the high voltage generator 174, and the guard trace is coupledto the negative side of the high voltage generator 174. Current measuredby the sensitive current meter 176 may be reported to the processor 172,which can correlate the measured current to a characterization of theparticulates in the exhaust gas to which the electrode 136 is exposed.More generally, the control and communications processor coordinates allmeasurements, does the analog to digital conversions, controls theoperation of the HV generator, processes the data, and communicates theresults via a CAN bus.

FIG. 6A depicts a schematic diagram of one embodiment of a sensorassembly circuit 182. This embodiment of the sensor assembly circuit 182does not include a guard trace. Generally, the current ISoot generatedby the sensor represents the charge loss current from the sensorelectrode 136. The resistance RLeak represents the leakage resistancebetween the electrode 136 and the sensor body 126, which is connected tothe exhaust pipe (see FIG. 1). The voltage VGO represents a groundoffset voltage between the exhaust pipe and the electronics ground. Themeasurement resistance Rm corresponds to a measurement current Imeasure.And the variable gain amplifier Amp amplifies the voltage drop over themeasurement resistance Rm caused by the measurement current Imeasure. Inthis embodiment, the measurement current is the sum of the leakagecurrent ILeak and the sensor current ISoot. In some embodiments, thisarrangement requires a very large leakage resistance RLeak in order toavoid false positives (e.g., 1 TOhm for <0.25 mg/m³ error).

FIG. 6B depicts a schematic circuit diagram of another embodiment of asensor assembly with a guard trace. The presence of the guard trace inthe illustrated circuit divides the leakage current ILeak over threeresistances, which include a first resistance RELeak between theelectrode 136 and the guard trace, a second resistance RGLeak betweenthe guard trace and a ground trace, and a third resistance RGG betweenthe ground trace and the sensor body 126 (and the connected exhaustpipe). The third resistance has no known impacts on the measurements ofthe sensor assembly.

The first resistance RELeak increases the load on the HV generator 174,while the sensor gain changes approximately linearly with the electrodevoltage between about 800-2,000V. Below about 800 MOhm for RELeak, theelectrode voltage will drop. The calculated gain error is only about 5%relative to an RELeak of about 150 MOhm, and goes down to about 0% asthe RELeak resistance value increases. In some embodiment, the electrodevoltage may be monitored between two thresholds, and known or calculatederror rates in the gain change for RELeak within that range may becompensated for. For example, gain changes for RELeak may be compensatedfor between about 40-800 MOhm.

The second resistance RGLeak is typically much larger than themeasurement resistance Rm, so the impact of the second resistance RGLeakmay be negligible for any values over about 100 kOhm. However, an inputoffset at the simplifier may create a false current through the secondresistance RGLeak. Thus, an amplifier with a very low offset may beused. For example, the amplifier may have an offset of about 20-25 μVover the target temperature range.

FIG. 7 depicts an exploded view of one embodiment of a heater assembly190 for use in a sensor assembly such as the sensor assembly 12 of FIG.2D. The heater assembly 190 is substantially similar to the heaterassembly 162 of FIG. 2D. The illustrated heater assembly 190 includesseveral layers of insulating materials 192 such as alumina. On the outersurfaces of the insulating layers 162, one or more ground traces 194 andone or more guard traces 196 are provided. The traces 194 and 196 may beany type of electrically conducting material (e.g., printed conductors)that is applied to the corresponding insulating surfaces. The heaterassembly 190 also includes heater conductors 198 and 200, including oneor more contact pad(s) and inner-layer innerconnect trace(s).

The ground trace 194 on the insulator layer 192 prevents ground offsetsbetween the electronics ground and the sensor body ground. In someembodiments, the ground trace 192 may be electrically coupled to theground side of the heater.

As explained above, the guard trace 196 effectively divides the leakageresistance RLeak into three separate resistances. This reduces errorsthat would otherwise occur due to the leakage current RLeak via bulkconductivity, surface contamination of the exposed insulating layers192, or condensation on any insulator layer 192. The guard traceelectrically extends through the system cabling via a shield conductor.In some embodiments, three-dimensional guard traces may be implementedwithin the sensor insulating layers to provide further protection.

FIG. 8A depicts a perspective side view of an assembled embodiment ofthe heater insulator assembly of FIG. 7. FIG. 8B depicts anotherperspective side view of an assembled embodiment of the heater insulatorassembly of FIG. 7.

FIG. 9 depicts results from modeling of one embodiment of a sensorassembly circuit which includes a guard trace 196. This modeling wasobtained from simulations performed with a test setup using an RELeakvalue of 10 GOhm, an RGLeak value of 40 MOhm (and down to 2.2 MOhm), andan RSoot value (representing the sensor) of 100 GOhm. The RELeakresistance exhibited about 100 nA leakage current, and the RSootexhibited about 10 nA with the guard trace 196 present. This is comparedwith about 12 nA without the guard trace 196.

FIG. 10 depicts one embodiment of a sensor electrode 136 with adeposited seed structure 240 of, for example, dendritic rhenium. Asensor electrode 136 that has not been operated previously may require,or benefit from, several minutes of operation in a gas stream containingparticles to reach a constant output with acceptable sensitivity.Initially, the observed current may be close to zero, then rises rapidlyto a value below the final sensitivity, and then rises slowly to thefinal sensitivity value. The timing of these changes in sensitivity aredue, at least in part, to the cumulative charge transfer effect ofparticles depositing on the exposed surface of the electrode 136 and/orthe corresponding surface of the inner baffle 140. Higher sootconcentrations during this initial period cause a faster rise to thefinal sensitivity value. In some embodiments, coating the positiveelectrode 136 of the sensor 12 with a seed structure duringmanufacturing can accelerate the ability of particles to deposit on theexposed surface area during operation of the sensor 12. DendriticRhenium (dendrites in the micron scale) is one example of a seedstructure that speeds up the startup process. In some embodiments, theuse of dendritic rhenium may accelerate reaching final sensitivity byabout 40-50%. In other embodiments, other types of seed structures maybe used.

FIG. 11A depicts an enlarged view of one embodiment of the seedstructure of FIG. 10. FIG. 11B depicts an enlarged view of anotherembodiment of the seed structure of FIG. 10 having additional particledepositions connected to the seed structure. FIG. 11C depicts a furtherenlarged view of the seed structure of FIG. 11B. In particular, FIGS.11B and 11C illustrate field enhancement and preferential sootdeposition primarily at the tips of dendritic rhenium.

FIG. 12 depicts one embodiment of a sensor electrode with a coupledstructure of nano-wires. In general, the nano-wires are characterized bymaterial spacing and lengths of nano-wires in approximately the micronrange, with tip diameters in approximately the sub-micron range. Thiscoupled structure of nano-wires may serve as an alternative type of seedstructure on the electrode 136 of the sensor. FIG. 13 depicts anenlarged view of another structure of nano-wires. In some embodiments,stable nano-wires are assembled during manufacturing. During operation,these nano-wires create the field enhancement to highly charge sootparticles. In this type of assembly, the amplification does notnecessarily come from agglomeration and breakoff of particles, butrather from the high charge between the nano-wire coated electrodes. Insome embodiments, the field amplification at the tips of the nano-wiresis chosen such that the repellant force experienced by a touchingparticle is always bigger than the intramolecular forces that allow asoot particle to stick to other materials like the nano-wires. Themagnitude of the field amplification is determined by the length/radiusratio of the nano-wires and their spacing. In other embodiments, otherfactors may influence the design and implementation of the nano-wires.Additionally, other embodiments may utilize other nano-structures thatare not necessarily classified as nano-wires.

In the above description, specific details of various embodiments areprovided. However, some embodiments may be practiced with less than allof these specific details. In other instances, certain methods,procedures, components, structures, and/or functions are described in nomore detail than to enable the various embodiments of the invention, forthe sake of brevity and clarity.

Although the operations of the method(s) herein are shown and describedin a particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operations may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be implemented in anintermittent and/or alternating manner.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The scope of theinvention is to be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A sensor comprising: an electrode for measuringcurrent due to movement of particulate matter relative to the electrode;and a seed structure deposited on the electrode, wherein the seedstructure comprises a plurality of elongated members extending outwardfrom the surface of the electrode, wherein the elongated members areconfigured to promote charge transfer to the particulate matter duringoperation of the sensor.
 2. The sensor of claim 1, wherein the seedstructure comprises dendric rhenium disposed on the surface of theelectrode.
 3. The sensor of claim 1, wherein the seed structurecomprises nano-wires disposed on the surface of the electrode.
 4. Thesensor of claim 1, wherein the elongated members are configured topromote local charge concentration at tips of the elongated members toenhance a strength of a local electrical field.
 5. The sensor of claim1, wherein the elongated members are configured to promote an attractionof the particulate matter to tips of the elongated members.
 6. Thesensor of claim 1, wherein the elongated members are configured topromote charge transfer to particles of the particulate matter duringoperation of the sensor.
 7. The sensor of claim 1, wherein the elongatedmembers are configured to promote charge transfer to agglomerates of theparticulate matter during operation of the sensor.
 8. A method formaking a sensor, the method comprising: providing an electrode formeasuring current due to movement of particulate matter relative to theelectrode; and depositing a seed structure on the electrode, wherein theseed structure comprises a plurality of elongated members extendingoutward from the surface of the electrode, wherein the elongated membersare configured to promote charge transfer to the particulate matterparticles and/or agglomerates during operation of the sensor.
 9. Themethod of claim 8, wherein the seed structure comprises dendric rheniumdisposed on the surface of the electrode.
 10. The method of claim 8,wherein the seed structure comprises nano-wires disposed on the surfaceof the electrode.
 11. The method of claim 8, wherein the elongatedmembers are configured to promote local charge concentration at tips ofthe elongated members to enhance a strength of a local electrical field.12. The method of claim 8, wherein the elongated members are configuredto promote an attraction of the particulate matter to tips of theelongated members.
 13. A method for using a sensor, the methodcomprising: passing exhaust by an electrode having a seed structuredeposited thereon, wherein the seed structure comprises a plurality ofelongated members extending outward from the surface of the electrode,wherein the elongated members are configured to promote charge transferto the particulate matter during operation of the sensor. measuringcurrent due to movement of particulate matter relative to the electrode;and correlating the measured current to a quantity of the particulatematter in the exhaust.
 14. The method of claim 13, wherein the seedstructure comprises dendric rhenium disposed on the surface of theelectrode.
 15. The method of claim 13, wherein the seed structurecomprises nano-wires disposed on the surface of the electrode.
 16. Themethod of claim 13, wherein the elongated members promote local chargeconcentration at tips of the elongated members to enhance a strength ofa local electrical field.
 17. The method of claim 13, wherein theelongated members promote an attraction of the particulate matter totips of the elongated members.
 18. The method of claim 13, whereinmeasuring the current further comprises measuring a change due to thecharge transfer from the electrode to particles of the particulatematter via the seed structure.
 19. The method of claim 13, whereinmeasuring the current further comprises measuring a change due to thecharge transfer from the electrode to an agglomerate of the particulatematter via the seed structure.